Tracks a Wheel Makes
Let your children drive toy vehicles through several different substances (water,
paint, mud, shaving cream, silly putty, etc) Talk to them about the different
tracks that is made in the different substances. 
Spokes on a Wheel
Make wheels with 1 - 7 spokes. Have your children put them in order
Rolling Jars
Get several Clear Jars of the same size. Put different small materials in each
one (a marble, crayon, block, pencil, etc.). Let the children roll them around
discuss with them how the items affect the jars ability to roll. 
Liscence Plate Matching Game
Materials:
- Large blank flash cards
- Markers or Paint
- Make up fake liscence plates to be used in a matching game. Have your children match the plates by number, state, color, etc.
Ramps
Make several ramps of different heights for your children to play with. Ask
them which one makes the cars go faster. See if they can tell you why. If not
tell them why.